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Stormy weather A cyclone the size of Europe lashed Saturn for five years, becoming the longest-lasting storm ever seen on the solar system's great planets, scientists say.

The cyclone, with a vortex 4000 kilometres wide, is being tracked by Spanish scientists with images from the Cassini spacecraft.

"Our observations make this cyclone the longest-lasting one ever seen on the giant planets of the solar system, Jupiter and Saturn," says the report's lead author, Teresa del Rio-Gaztelurrutia.

The discovery is surprising, for cyclones - where the wind spins in the same direction as the planet - usually do not last long, said the researcher, who led a team from the University of the Basque Country.

"We still know very little about these kinds of structures," she says.

Scientists began to track the cyclone in 2004 when Cassini beamed back first images of the planet.

They managed to analyse the horizontal and vertical structure of the cyclone, its circulation and its interaction with winds, using mathematical simulations.